The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of up to 12, typically 2, 3 or 4, but up to 5, 6, 7, or even 8 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butenes, pentenes and hexenes) can be converted by oligomerisation to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include alcohols, acids, detergents and esters such as plasticiser esters and synthetic lubricants. Industrial oligomerisation reactions are generally performed in a plurality of tubular or chamber reactors. Solid phosphoric acid, ion exchange resins, liquid phosphoric acid, sulphuric acid, molecular sieves, and zeolites, are known catalysts for oligomerisation.
Industrial hydrocarbon conversion processes employing zeolite catalysts typically run for several weeks or months before a catalyst change is required or a decommissioning of the reactor is needed. There is a general desire to increase run length to increase catalyst use and to reduce the amount of down time. However it is necessary to balance increasing the run length with the production of the desired product. Various attempts have been made to accomplish this, such as by the development of new catalysts or the control of temperature and pressure in the reactors as is described in PCT patent application PCT/EP2006/005851.
There are therefore continuing attempts to increase run lengths and these have led to olefin oligomerisation runs of several months. In industrial processes the feeds for the reactions are generally streams derived from catalytic or steam cracking, which may have been subjected to fractionation and other cleanup treatments. The nature of such refining activities is such that there will be variations in the composition of the feed. In addition, it may be desired to change the nature of the feed during a reactor run. The optimum catalyst activity and the optimum reaction conditions vary according to the composition of the feed. Furthermore, the reactions are exothermic and the exotherm also depends upon the nature and amount of olefin present in the feed. Butylenes, but especially isobutylene and propylene are particularly reactive feedstocks generating a large exotherm.
The feeds that are used for olefin oligomerisation are typically obtained from petroleum refining or petrochemical operations. In particular they are obtained from either the steam cracking or catalytic cracking of streams obtained from the processing of crude oil. The compositions of these oligomerisation feeds depends upon the feed to the cracking process and the cracking conditions that are employed. The composition of the oligomerisation feed and particularly the amount and nature of the impurities in the feed can have a significant impact on the conversion and selectivity of the oligomerisation reaction and can also effect the useful lifetime of the catalyst. Recently steamcrackers have been developed to process whole crude oils or the heavy fractions from crude oil distillations. Such feedstocks contain particularly high levels of nitrogen or sulphur. Examples of steamcracking process suitable for such feedstocks are described in WO 2005/113718 A2, WO 2004/005433 and WO 2004/005431, which are incorporated by reference. Alternatively the feeds may be produced by the conversion of oxygenates such as methanol to olefins.
It is well known that certain impurities such as sulphur containing contaminants and basic nitrogen containing species, including those compounds that are Lewis base, have an adverse effect on the useful lifetime of the catalyst and processes are employed to remove these contaminants from the feeds.
The present invention is concerned with oligomerisation processes that employ a zeolite oligomerisation catalyst in a tubular reactor and is particularly concerned with the provision of conditions which enhance the overall conversion and selectivity of the reaction and extend catalyst life. The present invention is concerned with reactions performed in tubular reactors, and although in no way limited to such the invention, is concerned with the production of octenes by the dimerisation of butene streams.
Octenes are used as feedstocks for hydroformylation for the production of C9 aldehydes, and upon hydrogenation of C9 alcohols, which are useful chemical intermediates such as raw materials for the production of plasticiser esters such as dinonyl phthalate. The plasticiser performance is dependent on the structure of the nonyl group which in turn is dependent upon the structure of the octene molecule from which the nonyl alcohol is produced. Octenes produced by the oligomerisation of olefines such as butene or butenes, including isobutene, tend to be a mixture of isomers of octene. Typically the isomers contain 1, 2 or 3 branches along the molecular backbone. The octenes are categorised by the average degree of branching, which is determined by first hydrogenating the mixture of isomers to remove unsaturation and then analysing the product of hydrogenation by gas chromatography for their individual isomers. The lower the degree of branching of the octene, the lower the viscosity of the plasticiser esters derived from the nonyl alcohol obtained from the octene, and the more effective the plasticiser. The invention is therefore concerned with improving the conversion and selectivity of a continuous olefin oligomerisation process, and in particularly to extending the period of time over which the improved conversion and selectivity can be achieved. In a preferred embodiment, the invention is concerned with improving the conversion and selectivity of a continuous process for the production of octenes by the dimerisation of butene. On butenes feed, the term overall selectivity relates to the ability to produce octenes, and the term structural selectivity relates to the ability to produce desirable octene isomers within the overall octene production.
Throughout this application, conversion is the percentage of fresh olefin feed that has reacted (and hence not retrieved anymore in the stream(s) leaving the process). It may be determined by making a material balance over the reactor/process and calculating % conversion of olefin as 100×(In−Out)/In.
Overall selectivity is typically defined as the production of the selected desired product(s). On (primarily) C4 feed these are typically the octene molecules, although the dodecenes may be included, and on (primarily) C3 feed these are the hexenes and dodecenes but even more importantly the nonene molecules. On mixed C4/C5 feeds these are the octenes and nonenes (and optionally the decenes), and on mixed C3/C4 feeds these are the hexenes, heptenes, octenes, nonenes (and optionally the decenes). Undesired typically are the heavier oligomers (typically the C10 or C11+ molecules, except for the tetramer (mainly C12) that is made from propylene), and the molecules that are not directly made by oligomerisation of fresh feed olefins, but made via a mechanism involving cracking to other than those fresh feed olefins. On C4 feeds those are typically everything but the octenes. On C3 feed, it are those other than C6/9/12s. Selectivity is expressed as a % wt found of the desired material relative to the amount of reaction products (excluding unreacted olefins and paraffins).
Structural selectivity is defined as the production of desired isomers within a mixture of isomers of a particular compound. This is determined by hydrogenation of the olefin to remove unsaturation which can interfere with a gas chromatogram and analysis of the product by gas chromatography. It is then possible to determine the number of molecules with 0, 1, 2 and 3 branches and from this the branching index may be calculated as the average number of branches per molecule. It is known that the structural selectivity and selectivity in the production of octenes from butene feeds can be improved by employing a process involving a relatively low conversion per pass combined with high recycle for high overall conversion, and this may be combined with increased reaction temperature. This technique is said to result in improved structural selectivity and a high overall conversion and selectivity to the preferred octene oligomer.
Throughout an extended oligomerisation production run, as the catalyst activity reduces, the reaction temperature is generally increased to maintain the desired level of conversion, and the reaction is terminated when a certain temperature representing the limits of the apparatus is reached for the desired level of conversion. Catalyst life or reactor run length is typically expressed as the amount (i.e. weight) of oligomer made per amount (weight) of catalyst, usually as ton/ton, lb/lb or kg/kg, and this provides a value that compensates for throughput variations, and this is a result of the material balance over the process throughout the run. The highest temperature that can be tolerated depends upon the equipment and the feed employed although we prefer to terminate with a temperature in the steam drum, in case one is provided, at less than 300° C., more preferably at less than 270° C. to avoid oligomer cracking reactions. This may allow the maximum temperature in the reactor tube or tubes to be as high as 310° C. or even 325° C., depending on the tube and reactor design.
Tubular oligomerisation reactors employing zeolite catalysts typically comprise one or more bundles of tubes also termed “reactor tubes”, mounted, preferably vertically, within a shell. The tubes are packed with the zeolite catalyst, typically in the form of pellets, and the feed containing olefin reactant is passed through the tubes in which it is oligomerised, typically from top to bottom. The length of the tube in industrial practice is generally from 2 to 15 meters, often from 3 to 14 meters, preferably from 5 to 12 meters, more preferably from 6 to 11 meters, yet more preferably from 8 to 10 meters. The diameter of the tube, the thickness of the walls of the tubes and the materials from which the tubes are made are important, since oligomerisation reactions are exothermic and it is important to dissipate the heat generated by the oligomerisation reaction. Accordingly, relatively small diameter, such as an external or outer diameter (OD) from 25 to 75 mm, tubes are preferred, more preferably 35 to 50 mm diameter (OD) tubes. The reactor tubes are preferably of high strength material and are thin walled and of a material with a high thermal conductivity. The high strength is required to withstand the high pressures that are generally used in the oligomerisation of olefins in a tubular reactor employing a zeolite catalyst. Duplex stainless steel is a preferred material for manufacture of the tubes. Higher strength steel and smaller tube diameters allow for smaller wall thicknesses. Duplex stainless steel and a 50.8 mm (2 inch) OD tube allow the wall thickness to be as little as 3 to 4 mm, leaving an internal diameter of the tube of 35-45 mm.
Any convenient number of tubes may be employed in a tubular reactor shell. Typically, operators use from 25 to 500 tubes per shell, arrayed in parallel. Preferred reactors contain about 77 tubes or 180 tubes per shell, although any number may be employed to suit the needs of the operator, e.g. 360 or 420. The tubes are preferably mounted within the shell and a temperature control fluid is provided around the outside of the tubes but within the shell to dissipate heat generated by the exothermic reaction that, in use, takes place within the reactor tubes. One reactor may comprise multiple bundles of tubes, for example up to 7 or 8, or even 9 bundles, and preferably, in use; the temperature of the fluid within the tubes in all the bundles in the same reactor is controlled by means of the same temperature control fluid system. Hot oil or boiling water, under pressure to control the temperature, may be used as the temperature control fluid. Reference to the temperature of tubular reactors as a whole herein is a reference to the temperature of the temperature control fluid, other discussions relate to temperature profiles within individual tubes.
Historically, oligomerisation reactions over acid catalysts are performed in the presence of water. The light olefinic feedstreams from refinery operations that are used for olefin oligomerisation typically contain water vapour from upstream in the process, because it is either added such as in steamcracking or catalytic cracking, or formed such as in the process of converting oxygenates to olefins. The feedstreams are therefore typically at their water dew point when they are condensed. This water will typically condense together with the light hydrocarbons, and there is usually sufficient water present to form free water that is then separated off by gravity. The liquid hydrocarbon stream containing the olefinic feed for oligomerisation is immiscible with water and has a lower density. It will tend to form a separate liquid layer above any liquid water phase. Due to some degree of water solubility, this layer will contain dissolved water. If a free water phase is formed, the level of dissolved water will be up to the solubility limit of water in the hydrocarbon stream. This limit is different for different hydrocarbon components, and therefore depends on the composition of the hydrocarbon stream.
U.S. Pat. No. 5,672,800 (WO93/16020) is concerned with the oligomerisation of olefins employing a zeolite catalyst, particularly the zeolite ZSM-22. U.S. Pat. No. 5,672,800 does not indicate the nature of the reactor that was used although it employs small quantities of materials and indicates that under the conditions employed in U.S. Pat. No. 5,672,800 conversion and catalyst life can be improved if the oligomerisation is performed in the presence of water. The compositions in the examples show a significant improvement in catalyst life when water is present. The catalyst life achieved on propylene using the techniques of U.S. Pat. No. 5,672,800 is 1240 weight of oligomer per unit weight of catalyst and 2500 weight of feed per unit weight of catalyst. According to U.S. Pat. No. 5,672,800, if the feed has a water content of from 0.05 to 0.25% molar, preferably at least 0.06% molar, based on the hydrocarbon content of the feedstock, the yields of the desired higher molecular alkene oligomers can be increased and the zeolite catalyst becomes deactivated more slowly. U.S. Pat. No. 5,672,800 specifies that if the water content is below 0.05 molar %, it should be increased. In Example 1 of U.S. Pat. No. 5,672,800 the moisture content of a feed having an initial water content of 0.02 molar % is hydrated to give a water content of 0.15 molar %, and the catalyst life is increased significantly, as is the propene conversion. U.S. Pat. No. 6684914 also hydrates the olefin feed to at least 0.05 mole % water. International Publication Number WO 2004/009518 suggests that the minimum water content of the hydrated olefin feed should be 0.005 wt %.
Although the use of water had been found to be beneficial, the water can interact with the zeolite to form oxygenates from the hydrocarbons in the feed. Although the reaction is not fully understood, it is believed that some of the olefins in the feed and the water react over the catalyst to form alcohols and ketones, which can be converted to acids, which have been found to cause severe corrosion downstream, particularly in the overhead of the first distillation tower, typically called the stabiliser column, and associated equipment for recycling unreacted feed molecules. This corrosion possibility requires equipment replacement and associated down time and/or the selection of more expensive corrosion resistant construction materials. PCT patent application PCT/EP2006/005852 relates to an oligomerisation process in which the olefin-containing feed stream contains less than 30 ppm wt of water.
The ExxonMobil Olefins to Gasoline (EMOGAS) process was described at the Annual Meeting of the National Petrochemical and Refiners Association, 13 to 15 Mar. 2005, at the Hilton Hotel, San Francisco, Calif., USA. The paper described olefin oligomerisation in a tubular reactor employing a zeolite catalyst, and specified that the reaction temperature is controlled with water that is fed on the shell side of the reactor. It is stated that the heat released due to EMOGAS reactions in the tubes evaporates water on the shell side. The temperature profile in the tubular reactor is said to be close to isothermal and the temperature is controlled via the shell side water pressure, which controls the temperature of evaporation, and also by the reactor feed temperature. The tubular reactors are said to usually operate at a pressure between 5.5 and 7.6 MPa (800 and 1100 psi) and at temperatures around 204° C. (400° F.).
The EMOGAS brochure also shows chamber-type reactors using interbed quench for temperature control. Adiabatic reactors in series for oligomerisation using interbed/interreactor cooling for temperature control are discussed in U.S. Pat. Nos. 4,487,985 or 4,788,366.
Reference in this specification to removal of heat from the (reactor) tubes of tubular reactors or temperature control of the (reactor) tubes is, in context, intended to mean removal of heat from the materials contained within the tubes where reaction takes place (generally comprising, in use, unreacted feed, reaction products and catalyst). It will be appreciated that the heat generation on the catalyst and heat removal from the tube wall may cause a radial temperature gradient through the cross-section of the tube, such that the centre of the tube may become significantly hotter than the wall of the tube. The larger the tube diameter, the larger this temperature gradient may be. One convenient way to remove the heat from the tubes and carry out the temperature control is to provide boiling water to generate steam within the reactor, on the shell side around the exterior of the tubes. This provides a good heat transfer coefficient on the shell side. If the present invention is performed in a chemical plant or a refinery, the steam generated by the oligomerisation process may be readily integrated into the steam system typically present at such sites. The reaction heat from oligomerisation may then be put to use in another part of the oligomerisation process, or with another process in the plant or the refinery, where heat input is required.
As already indicated, the oligomerisation of olefins over a zeolite catalyst is a highly exothermic reaction, particularly the oligomerisation of propylene and/or butylenes such as isobutylene. The high temperatures generated by the exotherm can lead to carbonaceous deposits on the catalyst caused by a build up of condensed, heavy hydrocarbons similar to asphalt. Such deposits are commonly termed “coke”, and may occur inside the zeolite pores and/or on the outer surface of the catalyst. This coke formation can lead to deactivation of the zeolite catalyst. In general, the higher the concentration of olefin in the feed, the higher will be the rate of heat release from the catalysed reaction, and hence the higher the temperatures that can be reached on and/or in the catalyst. Consequently there will be a higher rate of coke formation. This has placed a limit on the maximum concentration of olefin that can be tolerated in the feed. Since the oligomerisation reaction is highly exothermic, it is necessary to control the temperature and in a tubular reactor this is usually accomplished by encompassing a bundle of reactor tubes within a shell through which is passed a temperature control fluid. Conveniently, the temperature control fluid is oil (usually hot oil), or preferentially a boiling liquid because of the improved heat transfer this brings on the side of the boiling liquid. This boiling liquid may be an organic stream, preferentially a stream taken from another point in the process and its return stream, usually a mix of vapour and liquid, may be returned to another suitable point in the process. The reaction heat may as such be used as heat supply to a reboiler of a distillation tower. Most conveniently, the liquid is water, at least partially converting to steam in the reactor shell side. The water is conveniently supplied from a steam drum and the boiling temperature can then readily be controlled by varying the pressure in the steam drum. Conveniently, the steam drum collects the water/steam return stream from the reactor shell side, and provides the water supplied to the reactor shell side, most conveniently by thermosyphon action, avoiding the need for pumping or other means to drive the circulation. The steam generated by the reaction heat may be removed from the steam drum and may be put to use elsewhere. The temperature of the generated steam, or in such case of the temperature control fluid, exiting the reactor, is considered the temperature of the reaction, because it is the single most representative temperature for the reaction throughout the reactor.
When the conversion obtained in the reactor reduces, due to catalyst aging and/or coking, the reactor temperature is typically increased to compensate for the reduced catalyst activity. This is conventionally done by raising the steam pressure on the shell side of the reactor, which increases the temperature at which the heat exchange fluid boils. This procedure is called temperature ramping, and is typically limited up to a maximum temperature, when the reactor is typically taken out of service. Such temperature ramping is for instance disclosed in US 2006/199987, wherein a relatively constant olefin conversion is thereby ensured at a given weight hourly space velocity (WHSV).
U.S. Pat. No. 2,694,002 (Georges E. Hays) discloses a process for the polymerization of olefins in a first polymerization zone in the presence of a catalyst composed of oxides of silicon and aluminum in controlled ratio, the effluent thereof further threated in a second polymerization zone in the presence of a solid phosphoric acid type catalyst. In each zone, temperature, pressure and space velocity may be regulated, so that within a desired total conversion, the selectivity disadvantages of the two different catalyst systems are avoided and an improved selectivity to the desired gasoline components is achieved. U.S. Pat. No. 2,694,002 is not concerned with extending reactor runlength or catalyst life.
U.S. Pat. No. 2,440,822 (Karl H. Hachmuth) discloses a process for conducting catalytic reactions in heterogeneous catalyst portions used in parallel and that differ in activity. The process is described in more detail in connection with the polymerization of normally gaseous olefins to gasoline-range hydrocarbons, without specifying what type of catalyst should be used. The process of U.S. Pat. No. 2,440,822 controls the feed temperature to each of the adiabatic reactors in order to obtain the reaction zone temperature that is necessary in each of the reactors for achieving the desired conversion, which may be different and depend on the volume, the type and/or the age of the catalyst in the reactor. Further improved results may be obtained in the process of U.S. Pat. No. 2,440,822 by lowering the rate of flow to a relatively less active or relatively deactivated catalyst portion, because the lower flow rate decreases the average reaction zone temperature required in such a reactor and thereby decreases the rate of deactivation of the catalyst therein.
We have now found that the run life can be further extended by appropriate control of the space velocity of the olefin containing stream that is fed to the reactor. The space velocity is defined as the rate of feed supplied per hour divided by the weight of catalyst in the reactor.